The invention relates to homoepitaxial layer formation in the fabrication of semiconductor devices and, more particularly, to p-type epitaxial zinc oxide layer formation on a zinc oxide substrate.
Zinc oxide (ZnO) is a wide band gap semiconductor that is a promising material for the manufacture of radiation detecting devices (e.g., photoconductors, junction photodiodes, and avalanche photodiodes) operating in the ultraviolet, x-ray and gamma-ray regions of the electromagnetic spectrum and light emitting devices (e.g., light emitting diodes and laser diodes) operating in the ultraviolet and at blue wavelengths of the visible spectrum. Currently, radiation detecting and light emitting devices for these purposes are fabricated in an active layer of a compound semiconductor, typically gallium nitride (GaN), deposited heteroepitaxially on a single crystal substrate, such as sapphire. However, due to differences in lattice parameter and coefficient of thermal expansion, heteroepitaxy introduces a high density of dislocations that degrades the optical properties of the active layer material and leads to devices having an inferior performance. A buffer layer may be provided between the active layer and the substrate to alleviate the lattice mismatch. However, even the introduction of the buffer layer cannot prevent the introduction of a significant dislocation density into the active layer during deposition.
Zinc oxide has various advantages over GaN, in particular, for use as an active layer for fabricating radiation detecting devices and ultraviolet and blue light emitting devices. Among these advantages, ZnO has a significantly larger exciton binding energy than GaN, which suggests that ZnO-based lasers should have more efficient optical emission and detection. In addition, laser mirrors formed in GaN active layers on sapphire are more difficult and expensive to produce than ZnO-based lasers because the material is not cleavable. Furthermore, ZnO has a higher theoretical saturation velocity than GaN, potentially leading to faster device performance. Moreover, wafers of GaN, which would permit homoepitaxy of epitaxial GaN layers having a reduced defect density, are not commercially available at a reasonable price. Furthermore, because most common substrates, such as sapphire, used for heteroepitaxy of GaN active layers are electrically insulating, it is impossible to form backside ohmic contacts. Zinc oxide also has a better radiation-resistance than either gallium arsenide (GaAs) or GaN, which could find significance for radiation hardened electronics.
Molecular beam epitaxy (MBE) is a physical vapor deposition technique performed in ultra high vacuum in which a molecular beam of one or more constituent elements or compounds of interest is generated from pure elemental or compound material residing in a heated source, such as an effusion cell or Knudsen-type cell, and is directed toward an exposed surface of a heated substrate. The atoms or molecules comprising the molecular beam chemically combine at the surface of the substrate to form as a deposited epitaxial layer or thin film. Molecular beam epitaxy is particularly suitable for the deposition of high quality epilayers, heterojunctions, superlattices and multiple quantum wells on a single crystal substrate. In particular, epilayers deposited by MBE offer lowered defect densities than the substrate and more controlled doping compared to substrate doping.
A principle limitation to the use of active layers of ZnO for device fabrication has been the inability to produce reproducible, p-type conduction and, in particular, the inability to produce such p-type layers with techniques of MBE. Layers of p-type ZnO have been reported as successfully deposited heteroepitaxially on GaAs and sapphire substrates. However, devices formed in such active layers of p-type ZnO are deficient in their properties because of the significant differences in lattice parameter and coefficient of thermal expansion between the deposited layer and the substrate. For example, the lattice parameter of wurtzite ZnO along its c-axis is smaller than that of zincblende GaAs by about 9% and the thermal expansion coefficient of ZnO in a plane containing its a-axis is smaller than that of GaAs by about 26%. As a result, the heteroepitaxial deposition of ZnO on a substrate introduces a large lattice strain in the ZnO layer. This lattice strain precipitates lattice defects in the ZnO layer, which degrade the performance of devices formed in the ZnO layer and makes it difficult to manufacture practical ZnO-based light emitters and detectors by heteroepitaxy on such substrates. In addition, differences in coefficient of thermal expansion between the heteroepitaxial ZnO layer and the substrate cause thermal stresses that propagate dislocations, which ultimately results in dark lines or dead spots in degraded light emitting devices. As a result, radiation detecting devices and light emitting devices that incorporate active layers of p-type ZnO deposited heteroepitaxially on GaAs and sapphire substrates are generally of inferior quality and lack the stability to provide a usable device having a significant operational lifetime.
There is a need for a semiconductor structure having an epitaxial p-type ZnO layer of a reduced defect density for use in fabricating ZnO-based semiconductor devices and a method of forming such epitaxial p-type ZnO layers.
According to the present invention, a semiconductor structure is provided that includes an epitaxial p-type zinc oxide layer having an improved crystallinity. The semiconductor structure of the present invention is a single-crystal zinc oxide substrate having a surface and an epitaxial zinc oxide layer formed on the surface of the zinc oxide substrate. The zinc oxide layer includes an atomic concentration of a p-type dopant, such as nitrogen, sufficient to provide p-type conduction. In certain embodiments, the zinc oxide layer may include a compensating dopant or element, such as lithium, that electronically occupies excess donors present intrinsically in the zinc oxide layer. The compensating element enhances the effectiveness of the p-type dopant and, thereby, reduces the atomic concentration of the p-type dopant required to furnish the desired p-type dopant level.
The present invention further provides a method of fabricating a semiconductor device structure that includes providing a single-crystal zinc oxide substrate having a surface on which a zinc oxide layer is to be deposited and positioning the zinc oxide substrate in a molecular beam epitaxy system. A first flux containing zinc and a second flux containing oxygen atomic species, including radicals, neutrals, ions, and molecules, are formed in the molecular beam epitaxy system. The first and the second fluxes have relative magnitudes adequate to deposit zinc oxide. A third flux is formed in the molecular beam epitaxy system that contains atomic species of a p-type dopant present in an amount sufficient to provide p-type conduction in zinc oxide. The zinc oxide substrate is heated to a deposition temperature sufficient to promote crystalline deposition on the surface of the zinc oxide layer. Finally, the first, second and third fluxes are applied to the surface for a duration sufficient to epitaxially deposit thereon the layer of zinc oxide, doped with the p-type dopant in an atomic concentration sufficient to provide the layer with p-type conduction.
By virtue of the foregoing, there is provided an epitaxial p-type zinc oxide layer that eliminates the problems associated with the heteroepitaxial deposition of p-type zinc oxide on substrates, such as gallium arsenide, and that has a crystalline quality satisfactory for use as an active layer in a semiconductor device. The homoepitaxial deposition of the p-type zinc oxide layer on a single crystal zinc oxide substrate negates the introduction of stresses caused as a result of mismatch in the lattice constant and thermal expansion between the layer and the substrate. By doing so, the propagation of defects from the substrate into the p-type zinc oxide layer and through the p-type zinc oxide layer is lessened or prevented so as to significantly improve the crystalline quality of that layer.
The homoepitaxy of a p-type zinc oxide layer on a zinc oxide substrate has various other benefits. Among those benefits, zinc oxide has a relatively high exciton binding energy that pen-nits more efficient optical emission and detection, a higher (theoretical) saturation velocity to facilitate faster electronic devices, a commercially available native zinc oxide substrate for homoepitaxial deposition of thin films, and a lower cost for the basic materials used in fabrication. Zinc oxide also has a large photoconductivity and is an order of magnitude more radiation-resistant than most candidate semiconductor materials, including gallium nitride. Undoped zinc oxide substrates have n+-type conductivity, which facilitates the formation of backside ohmic contacts, to fabricate vertical device structures. The resultant ability to provide vertical device structures simplifies device fabrication and, in particular, significantly reduces the cost of fabricating radiation detecting devices and light emitting devices.